The present invention relates to a hydraulic brake booster for the production of a brake pressure in a motor vehicle brake system. The brake booster comprises a working piston for driving the master cylinder piston, a transmission piston preferably having a greater diameter than the working piston, a transmission pressure chamber filled with pressure fluid provided between the transmission piston and the working piston, a supply valve arrangement and a return valve arrangement leading to a reservoir associated with the transmission pressure chamber, a drive pressure chamber provided on the other side of the transmission piston, a control piston moveable out of a rest position by a brake pedal and a control valve arrangement disposed adjacent the transmission piston and control piston with an inlet valve and a discharge valve for connection of the drive pressure chamber to a pressure liquid source or a reservoir.
In a known brake booster of this type, such as disclosed in German Patent DE-OS No. 24 61 295, issued July 1, 1976, the transmission pressure chamber is provided to achieve a hydraulic transmission on the basis of different diameters of working piston and transmission piston. The working piston is in one piece connected to the master cylinder piston, or is connected to same via a pressure bar. In the rest position the transmission pressure chamber--just as the master cylinder pressure chamber--is connected to the reservoir via a compensating bore, which after a short stroke is closed and, therefore, serves as a supply--and return valve arrangement.
In a construction of this type the control piston and consequently the brake pedal have to move through a considerable idle path (travel of brake components) before the brakes are applied. This is due to the fact that the clearance at the individual brakes must be overcome and the compensating bores must be covered.
In a brake booster of the above type incorporating a transmission piston, it is already known from German Patent DE-OS No. 23 48 857 to divide the control piston into two parts which are shiftable relatively to one another by a predetermined measure, one of these parts being coupled with the brake pedal and operates the control valve arrangement and the other of these parts is exposed to the pressure in the drive pressure chamber and to a spring which acts opposingly. With this arrangement it is possible to fill, by slightly depressing the brake pedal, the drive pressure chamber sufficiently with pressure fluid and to shift the transmission piston correspondingly such that all idle paths are overcome. Thereby the second control piston part is shifted at the same time until it comes into engagement with the first control piston part. With further operation of the pedal the pressure in the drive pressure chamber increases corresponding to the pedal pressure. A valve with double function is connected into a connecting line between the transmission pressure chamber and the drive pressure chamber. This valve, on one hand, allows a supply of pressure fluid to the transmission pressure chamber and, on the other hand, a discharge of pressure fluid from the transmission pressure chamber if the working piston alone or in addition to the hydraulic pressure, shall be adjusted by the pedal force. Therefore, the opening in the case mentioned above is effected when a relatively high pressure limiting value is exceeded. This construction is especially complicated in the area of the control piston. Furthermore, the transmission piston must move through the total idle path and thus, its stroke must be correspondingly great. Additionally, at least with a greater idle path, the pedal must follow over a part of this idle path.